Use of surfactant in the preparation of modified sulfur and sulfur cement

ABSTRACT

Use of a non-ionic surfactant in the preparation of modified sulfur and/or modified sulfur cement that may or may not be modified sulfur concrete.

FIELD OF THE INVENTION

The present invention relates to the use of a non-ionic surfactant inthe preparation of modified sulfur. It also relates to modified sulfurand/or modified sulfur cement, such as modified sulfur concrete. It alsorelates to barriers and containment constructions comprising modifiedsulfur concrete, wherein the concrete comprises the non-ionicsurfactant.

BACKGROUND OF THE INVENTION

Sulfur has a number of allotropic forms, including orthorhombic,amorphous and monoclinic forms, with specific gravities of 2.07, 2.046,and 1.96 Mg/m³, respectively. Elemental (unmodified) sulfur undergoes acomplex transition in two steps between allotropic forms, from liquidsulfur above the melting point at 119.2° C. to solid sulfur at roomtemperature (or below 95.5° C.). Upon solidification sulfur initiallytakes a monoclinic β-phase. It undergoes 7% contraction in volumecompared with liquid sulfur. If elemental sulfur is used as a binderwith mineral aggregates to form a sulfur concrete material, thiscontraction leads to sub-pressure in pores and on surfaces.

The tensile capacity of sulfur, which is only 0.3-0.4 MPa, is notcapable of enduring the strain, and micro-cracking is inevitable. Thisopens up the elemental sulfur concrete material somewhat to moisturepenetration.

On further cooling of the sulfur, the monoclinic (β-phase) transformsinto the stable orthorhombic form (α-phase), at 95.5° C. This transitionis rather rapid (less than 24 hours) and leads to a further decrease ofvolume by 6%. It causes strain on the binder and cracking within thematerial, whether volume compensation has been made at solidification ornot. Historically, elemental sulfur concretes have failed (in themechanical sense, due to disintegration) when exposed to humidconditions, repeated cycles of freezing and thawing and immersion inwater.

In principle, there are two ways of treating this problem, relieving thematerial from imposed stress due to contraction; either by modifying thesulfur binder in such a way that it stays for a long time in the β-phase(the chemical way) or accepting the transition into the α-phase butpreventing, at least for a long time, the sulfur binder from formingmicro sulfur crystals which would cause contraction (the physical way).This is explained further in e.g. U.S. Pat. No. 4,293,463.

The chemical way is to combine sulfur with a modifying agent thatchemically modifies the sulfur in order to inhibit transformation to theorthorhombic structure. Suitable substances that may be used for thisinclude dicyclopentadiene, or a combination of dicyclopentadiene,cyclopentadiene and dipentene.

The physical way is to combine sulfur with a modifying agent thatphysically modifies the sulfur. Typically the modifying agent is anorganic plasticizer. Usually it comprises a polymer such as an olefinhydrocarbon polymer (e.g. RP220 or RP020 produced by Exxon Chemical orEscopol).

A durable sulfur concrete material not only requires a stable binder butalso a composition of aggregates and binder such that the full compositeremains stable and durable (e.g. it has limited absorption) underfluctuating temperature and moisture conditions.

Aggregates have been the focus of many efforts in seeking a durablesulfur concrete product. For example, moisture absorption can be limitedby the use of dense graded mineral aggregates, and proper compositiondesign with binder, mixing and consolidation. The selection of differentaggregates, which will be appropriate for each particular application,is necessary for a sulfur concrete material. To meet the requirement ofdurability, cleanliness and limits of harmful substances, the compositeaggregates must meet the ASTM C 33 specifications according to the ACICommittee 548. To determine an aggregate's suitability for a particularuse, it is recommended that preliminary testing be carried out forverification.

Corrosion resistant aggregates must be clean, hard, tough, strong,durable and free of swelling constituents. They should also resistchemical attacks and moisture absorption from exposure to acid and saltsolutions. Moisture absorption and dissolution losses should not exceed1% in a 24 hour period.

When clay is contained within solidified sulfur concrete, the clay isbelieved to have an absorptive capacity, which will allow water topermeate through the material. When clay absorbs water, expansionoccurs, resulting in deterioration of the product. Thus, clay-containingaggregates should not be used in producing sulfur concrete withouttreatment for limiting the swelling capacity.

Sulfur concrete is prepared in a different way from Portland cementconcrete. New gradation designs have been developed based on thetechnology for asphalt concrete. The intention was to develop aggregatemixtures with maximum density and minimum voids in the mineralaggregate, so that less sulfur is needed to fill the voids of themixture. The optimum range for the sulfur content of the sulfur concreteis slightly less than the amount necessary to fill the aggregate to 100%saturation, yet high enough to keep the final void content less than 8%.This, in most cases, results in higher strength materials, becauseimproved aggregate contact means less shrinkage after solidification.

The mineral filler forms, with the binder, the paste which coats andbinds the coarse and fine aggregate particles to produce a strong anddense product. Fillers should (1) control the viscosity of the fluidsulfur-filler paste, workability and bleeding of the hot plasticconcrete; (2) provide nucleation sites for crystal formation and growthin the paste and minimize the growth of large needle-like crystals; (3)fill voids in the mineral aggregate, which would otherwise be filledwith sulfur, reducing hardening shrinkage and the coefficient of thermalexpansion; and (4) act as a reinforcing agent in the matrix to increasethe strength of the formation.

Therefore, to meet the above mentioned functions, the filler must bereasonably dense-graded and possibly finely divided, so as to provide alarge number of particles per unit weight, especially to meet thefunction (2) as described above (provision of nucleation sites).

As the preceding discussion indicates, much research has focussed onphysically controlling the adverse effects of sulfur concrete bycontrolling the aggregates. Such physically controlled materials are notalways available, for instance in arid lands.

Various uses have previously been suggested for sulfur concretes,including commercial applications such as the construction of chemicalvats, the encapsulation of radioactive waste or mixed wastes in sewageand brine handling systems, and electrolytic baths. Sulfur concreteshave also been used by the Corps of Engineers in repairing dams, canallocks, and highways. The use of sulfur concrete materials as barriersystems has been accepted by the US Environmental Protection Agency.

Various uses have also previously been suggested for modified sulfurconcretes, including rigid concretes, flexible paving, spray coating,grouts and the temporary containment of corrosive compounds such asacidic and salt solutions.

However, it has not been suggested to use such modified sulfur concretesto restrict permeation over a long time period. The restriction ofpermeation over a long time period may be useful in, for instance, wastecontainment. Thus, hazardous waste requiring long-term containment callsfor a containment construction comprising a barrier that restrictspermeation over a long time period. In this instance, the barrier canhelp to protect subsurface soils and groundwater from contamination bytoxic substances in the hazardous material due to leaching and movementby ground water action. It can also provide a means for isolation andconfinement of the toxic substances within their storage or disposalhost environment.

Materials that are currently being used for this purpose includematerials that have mainly been used in engineering practice such ashydraulic cement, clay based soil, thermoplastic organic binders andthermosetting organic binders. These materials are being utilized ascontainment barrier systems around hazardous materials being stored ordisposed of in underground or surface excavations. Clay based soilbarriers are generally used because of their low hydraulic conductivity.In arid land regions, where the clay materials are unavailable,prefabricated synthetic materials in combination with bentonite aregenerally used. It has been proposed to use ordinary and special cementsand concretes but this approach has not proven entirely satisfactory.

Among the possible desirable properties for a barrier are the following.It should (1) form an impervious barrier to the action of ground andsaline waters; (2) have a low leaching rate, particularly by ground orsaline waters; (3) be relatively inert; (4) have good resistance tochemical and physical degradation and biological processes; (5) becompatible with the containment construction and any uncontainedhazardous material in the host environment; (6) exhibit a long-termsatisfactory behaviour as a barrier or backfill material in the storageor disposal environment; (7) be in plentiful supply and at a reasonablecost; and (8) be easy to handle and control from an operating andmanufacturing point of view. Materials suitable for use as barriers forhazardous waste require hydraulic conductivity in the order of 10⁻⁹ m/sor less.

Some researchers have used sulfur to solidify liquid low-levelradioactive waste. The solidified material is disposed in a landfillwhich uses e.g. clay based barrier systems and geosynthetics. Thus, theleaching of metals from the sulfur-based material has not been a majorconcern. In this scenario, one would expect that metals will be leachedout from the sulfur matrix but be contained by the barrier system. Inmost cases the sulfur matrix has been prepared from molten sulfurwithout any chemical additives.

When researchers have attempted to use chemical additives for sulfurmodification, durability of the sulfur concrete has been questionablebecause of the type of chemicals used. Long-term durability to chemicalattacks and temperature has been examined but the necessary level ofsatisfaction for engineering applications has not been met.

SUMMARY OF THE INVENTION

It has surprisingly been found that surfactants which are non-ionic mayadvantageously be used in the modification of sulfur. Such surfactants,when used in combination with a mixture of oligomeric hydrocarbons,enable the production of modified sulfur that is useful, for instance,in the preparation of modified sulfur concrete. Modified sulfur concreteobtainable using a non-ionic surfactant in combination with a mixture ofoligomeric hydrocarbons has surprisingly been found to possess excellentproperties in terms of strength, durability and leachability, includinga hydraulic conductivity in the order of 10⁻¹³ m/s. The use of suchmodified sulfur concrete is particularly advantageous in arid areas,where materials such as clay and other fine-grained soils are notreadily available and are therefore usually expensive because they mustbe transported from remote locations. The excellent properties of themodified sulfur concrete of the present invention are also advantageousfor waste containment, e.g. for containing hazardous chemical orradioactive waste.

Accordingly, the present invention provides the use of a non-ionicsurfactant in the preparation of modified sulfur, and/or modified sulfurcement that may or may not be modified sulfur concrete.

The present invention also provides a process of producing modifiedsulfur, which process comprises mixing elemental sulfur, a mixture ofoligomeric hydrocarbons and a non-ionic surfactant to produce a mix.

The present invention also provides modified sulfur, which comprisessulfur, a mixture of oligomeric hydrocarbons and a non-ionic surfactant,and the use of such modified sulfur in the preparation of modifiedsulfur cement which may or may not be modified sulfur concrete.

The present invention also provides a process of producing modifiedsulfur cement (or, if aggregates are present too, modified sulfurconcrete), which process comprises mixing elemental sulfur and themodified sulfur of the present invention.

The present invention also provides modified sulfur cement (or, ifaggregates are present too, modified sulfur concrete), which compriseselemental sulfur and the modified sulfur of the present invention. Themodified sulfur concrete of the present invention is a high strength,essentially impermeable, acid and salt resistant material that issuitable for use in very aggressive environments. It provides along-term, cost effective alternative to Portland concrete whereprotection by acid brick, coatings, linings or other protective systemsis required in highly corrosive environments. A further advantage of themodified sulfur cement of the present invention is that it hasthermoplastic properties. Thus, when it is heated above its meltingpoint, it becomes liquid, and can be mixed with aggregates such as sand,soil or wastes, to produce modified sulfur concrete. On cooling the mixre-solidifies to form a solid monolith. Full strength is achieved inhours rather than weeks as compared to hydraulic cements. Further, nochemical reaction is required for setting as in hydraulic cements. Thisminimizes incompatibilities between binder and aggregate. In arid lands,where evaporation is very high, the use of hydraulic cement (for whichthe use of water is needed to hydrate the cement and produce a solidmatrix) is hindered by the lack of water. As a result public workssuffer from excessive shrinkage and loss of strength. However, sulfurcement production does not require water.

The present invention also provides the use of the modified sulfurconcrete of the present invention as a barrier to restrict permeation ofmatter, and a barrier suitable for restricting permeation of matter,which barrier comprises the modified sulfur concrete of the presentinvention.

The present invention also provides a containment construction suitablefor containing matter over a long time period, which constructioncomprises one or more barriers of the present invention.

The use of modified sulfur concrete of the present invention as abarrier to restrict permeation of matter, e.g. in a containmentconstruction, is particularly advantageous in arid land because of thehigh temperature environment. It is also advantageous in view of thefact that clay materials are poorly available and subsurface soils inarid lands have a high hydraulic conductivity (in the order of 10⁻⁵m/s). In addition, it is advantageous because synthetic materials areexpensive, particularly in view of the quality control that would beneeded, and the risk of accidents (e.g. material puncture) duringconstruction that could lead to the escape of polluting leachetes.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a mechanism for sulfur polymerisation.

FIG. 2A and FIG. 2B give SEM images showing the difference between pureelemental sulfur and sulfur modified with bitumen, at the same heatingand cooling conditions.

FIG. 3 is an SEM image illustrating how sulfur and modified sulfur bind,coat, and penetrates deep and between the aggregates.

FIG. 4 shows 28-Day immersion test results obtained by subjectingsamples of concrete structures of the present invention to differentsolution environments and different temperatures.

FIG. 5 shows 1 year immersion test results obtained by subjectingsamples of concrete structures of the present invention to differentsaline solutions, at the same temperature.

FIG. 6 is an SEM micrograph of a fracture 1 cm from the surface of asample of the concrete of the present invention after immersion for oneyear in distilled water, showing a different coating of aggregateparticles with sulfur.

FIG. 7 shows variations in the amount of sulfur leached from concretesamples with time and solution pH.

FIG. 8 shows variations in the amount of Ca, Mg, Al, and Fe leached fromconcrete samples with time and solution pH.

FIG. 9 shows variations in the amount of sulfur leached from concretesamples with time and temperature.

FIG. 10 shows variations in the amount of Ca and Mg leached fromconcrete samples with time and temperature.

FIG. 11A shows the design of a typical hazardous waste containmentconstruction.

FIG. 11B shows the design of a typical hazardous waste containmentconstruction that is for use in arid land.

FIG. 11C shows the design of a new containment construction provided bythe present invention.

FIG. 12 shows a DSC thermogram for modified sulfur of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

By “non-ionic” it is meant that the surfactant does not contain a headwith a formal net charge.

The non-ionic surfactant is preferably an alkylaryloxy polyalkoxyalcohol.

The alkyl group in the alkylaryloxy polyalkoxy alcohol typically has upto 12 carbon atoms, such as 2 to 10, or 4 to 8 carbon atoms. It can bestraight, though preferably it is branched. Preferably it isunsubstituted. Typically it is octyl, more typically iso-octyl.

The aryl group in the alkylaryloxy polyalkoxy alcohol typically containsfrom 6 to 10 carbon atoms. It can be a monocyclic ring, for examplephenyl, or, unless otherwise specified, may consist of two or more fusedrings, for example naphthyl. Preferably it is unsubstituted. Typicallyit is phenyl.

The alkoxy group in the alkylaryloxy polyalkoxy alcohol typicallycontains 1 to 4 carbon atoms, such as 2 or 3 carbon atoms. Preferably itis ethoxy.

The terminal alcohol moiety in the alkylaryloxy polyalkoxy alcoholtypically has the same number of carbon atoms as the repeated alkoxygroup. Preferably it contains 1 to 4 carbon atoms, such as 2 or 3 carbonatoms. Most preferably it is has 2 carbon atoms.

The polyethoxy section typically contains an average of 7 to 40 ethoxyunits, preferably less than 30, more preferably less than 20, such asless than 10. In one embodiment the average number of ethoxy units is 9.In another embodiment the polyethoxy section contains an average of 5 to15 ethoxy units.

The alkylaryloxy polyalkoxy alcohol can be a copolymer containingdifferent types of alkoxy units, e.g. it may comprise a mixture ofethoxy and propoxy units.

Typically the alkylaryloxy polyalkoxy alcohol is an alkylphenoxypolyethoxy ethanol. Preferably the alkylphenoxy polyethoxy ethanol hasthe average formula C_(r)H_(2r+1)(C₆H₄)O(CH₂CH₂O)_(s)CH₂CH₂OH, wherein ris from 4 to 12 and s is from 7 to 40. r is preferably from 5 to 10,such as 7 to 9. In one embodiment r is from 4 to 8. Typically r is 8. sis preferably less than 30, more preferably less than 20 and typicallyless than 10. In one embodiment s is 9.

In one preferred embodiment the surfactant is iso-octylphenoxypolyethoxy ethanol. The non-ionic surfactant may, for instance, beTriton X-100®, which is manufactured by Rohm and Haas Company,Philadelphia, Pa.

The non-ionic surfactant is typically used in combination with a mixtureof oligomeric hydrocarbons.

Various species may be present as oligomeric hydrocarbons. The mixtureof oligomeric hydrocarbons typically comprises one or more polycyclicaromatic hydrocarbons. Thus, the mixture of oligomeric hydrocarbons canbe a composition comprising one or more polycyclic aromatichydrocarbons.

The polycyclic aromatic hydrocarbons for use in accordance with thepresent invention include, for instance, naphthalene, anthracene,phenanthrene, fluoranthene, naphthacene, chrysene, pyrene, triphenylene,benzofluorathene, perylene, pentacene, corannulene, benzo[a]pyrene,coronene and ovalene. Typically, the polycyclic aromatic hydrocarbonsare one or more selected from naphthalene, anthracene, phenanthrene,fluoranthene, chrysene, pyrene, benzofluorathene, perylene andbenzo[a]pyrene. In one embodiment phenanthrene and pyrene are used.Typically phenanthrene is used.

The polycyclic aromatic hydrocarbons for use in accordance with thepresent invention are unsubstituted or substituted. When substituentsare present they are typically hydrocarbon substituents, such as alkyl,alkenyl and alkynyl substituents, though typically they are alkyl. Thehydrocarbon substituents generally have 1-10 carbon atoms, typically 1-6or 1-4 carbon atoms. The hydrocarbon substituents may be straight orbranched. Preferred examples of the hydrocarbon substituent are methyl,ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl and t-butyl. Morepreferred are methyl and ethyl. Most preferred is methyl.

The mixture of oligomeric hydrocarbons typically comprises one or moreasphaltenes. Thus, the mixture of oligomeric hydrocarbons can be acomposition comprising one or more asphaltenes.

The asphaltenes for use in accordance with the present invention aretypically alkylated condensed aromatic rings. The asphaltenes aretypically insoluble in n-heptane insoluble but soluble in toluene. Theasphaltenes typically have a range of molecular masses from 400 to 1500units. The most common molecular mass is typically around 750 units. Asuitable method for checking molecular mass is ESI FT-ICR MS.

The mixture of oligomeric hydrocarbons typically comprises one or morealkanes. Thus, the mixture of oligomeric hydrocarbons can be acomposition comprising one or more alkanes.

The alkanes for use according to the present invention can have varyingnumbers of carbon atoms, e.g. alkanes with up to 20 carbon atoms, 20-35carbon atoms and/or 35 carbon atoms and above. The alkanes can bestraight. Alternatively they can be branched, e.g. iso-alkanes.

In one embodiment the alkanes can be or include cycloalkanes, i.e.naphthenes. Naphthenes can be present instead of acyclic alkanes thoughtypically both are present. The naphthenes can contain, for instance 3or more rings, such as 4 or more, or 5 or more. In one aspect of theinvention they contain less than 40 rings, such as less than 30, lessthan 20 or less than 10. The naphthenes can be unsubstituted orsubstituted with alkyl groups, wherein the alkyl substituents are thesame as described above for the polycyclic aromatic hydrocarbons.

The mixture of oligomeric hydrocarbons typically comprises one or moreresins. Thus, the mixture of oligomeric hydrocarbons can be acomposition comprising one or more resins.

The mixture of oligomeric hydrocarbons may or may not comprise traces ofmetals such as iron, nickel and vanadium, and/or traces of non-metalelements such as oxygen, nitrogen, sulfur, phosphorous and halogens.When these non-metal elements are present they can appear at appropriateplaces within the hydrocarbon structures of the mixture of oligomerichydrocarbons.

Preferably, the mixture of oligomeric hydrocarbons has an average degreeof polymerization of 8 to 12, typically around 10. It is also preferredthat the mixture of oligomeric hydrocarbons is a composition comprisingone, more than one or all of polycyclic aromatic hydrocarbons,asphaltenes, alkanes (typically both acyclic and cyclic) and resins.Typically the mixture of oligomeric hydrocarbons is a compositioncomprising all of these, such as bitumen.

Bitumen is a black, oily, viscous material that is a naturally-occurringorganic by-product of decomposed organic materials. It is obtainablefrom the bottom most fractions obtainable from crude oil distillation.It is too thick and sticky to flow, wholly soluble in carbon disulfide,and mostly made up of highly condensed polycyclic aromatic hydrocarbons.

The term “modified sulfur” refers to sulfur in which either (a) theamount of sulfur in the α-phase is lower than that which would beobserved if molten elemental sulfur was allowed to cool to roomtemperature on its own, or (b) the amount of sulfur in the α-phase whichis present in the form of micro crystals is lower than that which wouldbe observed if molten elemental sulfur was allowed to cool to roomtemperature on its own. Typically in the modified sulfur the proportionof the sulfur that is not present in the α-phase (i.e. the orthorhombicform) is at least 5%, such as at least 10% or at least 20%. Moretypically it is at least 30% or at least 40%. Preferably in the contextof the present invention the modified sulfur satisfies both (a) and (b),and the proportion of the sulfur that is not present in the α-phase(i.e. the orthorhombic form) is present predominantly as polysulfideinstead. Thus, preferably the degree of polymerisation in the modifiedsulfur is at least 10%, such as at least 20 or 30. Typically it is atleast 40%.

The term “modified sulfur cement” refers to sulfur cement that comprisesmodified sulfur. The term “modified sulfur concrete” refers to sulfurcement that further comprises aggregates.

The modified sulfur of the present invention typically comprises atleast 90%, preferably at least 95%, typically less than 98% by weight ofsulfur. Preferably it comprises 95-97.5% by weight of sulfur. Themodified sulfur of the present invention typically comprises 0.01-0.05%by weight of the non-ionic surfactant, preferably 0.02-0.04% such as0.02-0.03% or around 0.025%. The modified sulfur of the presentinvention typically comprises 1-5% by weight of the mixture ofoligomeric hydrocarbons, preferably 2-4% such as 2-3% or around 2.5%.

Preferably the modified sulfur of the present invention comprises95-97.5% by weight of sulfur, and 2.5-5% by weight of the total ofbitumen and non-ionic surfactant components, based on the total weightof the modified sulfur.

In the process of producing modified sulfur, the preferred amounts ofstarting material to use essentially correspond to the amounts that arepreferably present in the modified sulfur of the present invention. Forexample, in a preferred aspect the process of producing the modifiedsulfur comprises mixing elemental sulfur, bitumen and a surfactantwherein the elemental sulfur accounts for 95-97.5% by weight of themixture and the total of the bitumen and surfactant components accountsfor 2.5-5% by weight of the mixture.

The reaction time in the process of producing the modified sulfur isusually at least 30 minutes, though typically is less than 3 hours, moretypically less than 2 hours. Preferably the reaction time ranges from45-60 minutes. Reaction temperatures of 120-150° C. are generally used,preferably 130-140° C. Typically temperatures of 135-140° C. are used.Most preferably a temperature of around 140° C. is used.

After heating and mixing, the process preferably comprises cooling themixture. The cooling can be carried out by simply leaving the mixture tocool to the surrounding temperature of its own accord or by activelyinducing and/or controlling the cooling in some way. Typically a coolingrate of less than 5° C. per minute, such as less than 2 or 3° C. perminute, preferably around 1° C. per minute is employed. Generally thiscooling rate is used throughout the entire cooling process. Thetemperature measured to calculate the cooling rate is the meantemperature for the whole of the concrete.

In the casting step, the temperature of the mould is preferably higherthan or equal to the temperature of the mixture being placed in it.Typically the temperature of the mould is higher than or equal to themost recent mixing temperature. In another preferred embodiment,vibration of the mixture can be used to produce a highly dense modifiedsulfur concrete. A curing time of 1 day is generally required before themodified sulfur concrete is suitable for contact with water and/or e.g.any waste that it is intended to restrict permeation of.

Suitable methods for forming modified sulfur cement are described inMohamed et al, Compositional control on sulfur polymer concreteproduction for public works, The Seventh Annual UAE University ResearchConference Proceedings 2006, Sat April 27 15:45-16:00, Eng 131-Eng 140.

In one preferred embodiment of the present invention, the modifiedsulfur is obtainable by a process of the present invention as definedherein.

In the process of producing modified sulfur, the non-ionic surfactant,in combination with the mixture of oligomeric hydrocarbons, physicallymodifies the sulfur by inducing sulfur polymerization. Thus, theresulting modified sulfur cement comprises polymerized sulfur. Whenpolymerized sulfur is present the sulfur phase transformation (β to α)still occurs during cooling, but the polymerised sulfur acts as acompliant layer between the sulfur crystals, and so serves to mitigatethe effect of the phase transformation.

In a preferred embodiment of the present invention the modified sulfurcomprises 45-65%, preferably 50-60% and typically around 55% by weightof monoclinic sulfur and 35-55%, preferably 40-50% and typically around45% by weight of polysulfide, based on the total weight of the sulfurcomponent.

The degree of polymerization can be confirmed by analyzing the fractionof the product that is insoluble in carbon disulfide (CS₂) by columnchromatography (HPLC Agilent 1100; column PLgel Mixed C, 300*7.5 mm*5μm, flow rate of 1 ml/min in chloroform, at room temperature 24° C.).

Typically both low and high molecular weight fractions of polysulfidesare present in the modified sulfur of the present invention. The weightaverage molecular weight of the polysulfides is preferably from10,000-30,000, typically 15,000-20,000. The average number molecularweight of the polysulfides present in the modified sulfur is typically200-500, preferably 300-400. The poly-dispersability index of thepolysulfides present in the modified sulfur, which is a reflection ofthe product molecular weight distribution, is preferably from 3-7, morepreferably from 4-6, and typically 5.

In preparing the modified sulfur of the present invention, reaction ofthe non-ionic surfactant and the mixture of oligomeric hydrocarbons withthe elemental sulfur (i.e. the degree to which they can disperse in eachother) depends on how they interact. Types of interaction are: pi-pibonding, polar or hydrogen bonding (polar interactions of hetero atoms)and Van Der Waals forces. Preferably the non-ionic surfactant is used incombination with bitumen, which when combined with sulfur allows theproduction of a homogeneous, self-compatible mixture consisting of avariety of molecular species that are mutually dissolved or dispersed.Typically this combination contains a continuum of polar and non-polarmaterial. This leads to areas of order or structure of polysulfides inthe modified sulfur, depending on the amount of the polymer present, thereaction time, the reaction temperature, and the cooling rate.

At heating temperature <140° C., elementary sulfur forms polysulfides.The mechanism believed to explain this process is depicted in FIG. 1.Essentially it takes place through initiation and propagation steps.

Initiation: cyclo-S₈→chain-S₈:  (1)

Propagation: chain-S₈:→S_(poly):  (2)

Sulfur undergoes a liquid-liquid transition, usually interpreted as thering opening polymerization of elemental sulfur S₈. An increase intemperature is accompanied by an increase in motion and the bond withinthe ring becomes strained and finally breaks. The covalent bond breaksequally in half, so a di-radical is formed. Ring opening gives rise totriplet di-radical chains. Polymerization then occurs to form longchains.

The modified sulfur of the present invention can be used in thepreparation of modified sulfur cement, such as modified sulfur concrete.The process of producing modified sulfur cement comprises mixingelemental sulfur and the modified sulfur of the present invention asdefined above. Preferably the process also comprises mixing an aggregatewith the elemental sulfur and modified sulfur. In this case the modifiedsulfur cement product is modified sulfur concrete.

Thus, the modified sulfur cement of the present invention preferablycomprises one or more types of aggregate. This further improves thestrength and extends the utilization of the modified sulfur cement.Thus, the aggregates act as physical stabilizers. Modified sulfur cementcomprising aggregates is modified sulfur concrete.

An aggregate is typically a strengthening material. Generally anymaterial may be used as an aggregate so long as it does not adverselyreact with any of the other components of the sulfur cement. Thus, anaggregate serves a physical purpose in the cement rather than a chemicalone and accordingly may comprise any inert particles so long as they areof appropriate size. Appropriate sizes are 0.01 to 1 mm, preferably 0.05to 0.5 mm.

One possible type of aggregate is a waste material. This brings theextra advantage of finding a beneficial use for by-products of otherindustries that are generally unwanted and may otherwise requiredisposal. Examples include fly ash, slags from iron and steel making,non-ferrous slags, domestic refuse incinerator ash, overburdenmaterials, dredged silts, construction rubble, waste water treatmentsludges, and paper mill sludges. As these materials may include traceelements of potential pollutants and/or heavy metals (that can posevarious environmental risks), care should be given before using them toassess the possible hazard expected during infiltration conditions.

The present invention has the advantage that there is no need to controlthe gradation of the aggregates. Thus, cheaper starting materials can beused. The use of aggregates can also further reduce costs, because cheapwaste material can be used. Also, it adds significant strength thanks tothe resulting grain structure.

Preferably, fly ash is used as an aggregate, i.e. the aggregatecomprises fly ash. Fly ash is the ashy by-product of burning coal, alsowell-known as coal ash. Fly ash superior waste, which is a waste productof the nuclear industry, may also be used. Physically, fly ash is a veryfine, powdery material. It is predominantly silica, with particles inthe form of tiny hollow spheres called ceno-spheres. Type C fly ash istypically used, though other types such as type F may also be used.These two types of fly ash have pozzolanic properties, but type C flyash is preferred because in the presence of water it hardens and gainsstrength over time. If the aggregate comprises fly ash, the fly ashtypically accounts for at least 30%, preferably at least 40%, typicallyat least 50% of the aggregate.

Preferably, sand is used as an aggregate, i.e. the aggregate comprisessand. Sand is naturally occurring, finely divided rock, comprisingparticles or granules. The most common constituent of sand is silica(silicon dioxide), usually in the form of quartz, which because of itschemical inertness and considerable hardness, is quite resistant toweathering. If the aggregate comprises sand, the sand typically accountsfor at least 25%, preferably at least 35%, typically at least 45% of theaggregate.

As is evident from the above discussion, many different types ofcompound may be used as aggregate, provided they do not interfere withthe concrete formation process. To this end, the present invention hasthe advantage that it allows the use of undesirable materials, which areboth cheap and may also otherwise require disposal, with an associatedenvironmental and economical cost.

In one embodiment the present invention provides modified sulfurconcrete wherein the aggregates comprise hazardous waste. Thus, theconcrete, once set, has the hazardous waste embedded within it, i.e. thewaste is contained by solidification.

In the modified sulfur concrete of the present invention, the amount ofaggregate is generally at least 30%, preferably at least 40%, morepreferably at least 50%, more preferably still at least 60% by weightbased on the total weight of the resulting modified sulfur concrete. Theamount of aggregate may be up to 85% or even up to 90 or 95% by weightbased on the total weight of the resulting modified sulfur concrete.However, typically the amount of aggregate is less than 85%, preferablyless than 80%, more preferably less than 75%, more preferably still lessthan 70% by weight based on the total weight of the modified sulfurconcrete. Typically the amount of aggregate is 50 to 85%, morepreferably 60 to 70% based on the total weight of the modified sulfurconcrete.

In the modified sulfur cement of the present invention, the amount ofelemental sulfur is generally at least 97%, preferably at least 98%,such as around 98.5 or 99% by weight based on the total weight of themodified sulfur cement.

In the modified sulfur concrete of the present invention, the amount ofelemental sulfur is generally at least 20%, preferably at least 25%,more preferably at least 30% by weight based on the total weight of themodified sulfur concrete. The amount of elemental sulfur is generallyless than 50%, preferably less than 45%, more preferably less than 40%by weight based on the total weight of the modified sulfur concrete.

In the present invention, the modified sulfur for use in preparing themodified sulfur cement (e.g. modified sulfur concrete) of the presentinvention will inevitably contain a certain amount of “unmodified” (i.e.unpolymerized) sulfur. However, when the amount of elemental sulfur inthe modified sulfur cement (such as the modified sulfur concrete) isreferred to herein, it refers to the amount of sulfur derived fromelemental sulfur rather than from modified sulfur as a startingmaterial.

In the modified sulfur cement of the present invention (which ispreferably concrete), the amount of modified sulfur is generally atleast 0.1%, preferably at least 0.25%, more preferably at least 1% byweight based on the total weight of the modified sulfur cement. Theamount of modified sulfur is generally less than 3%, preferably lessthan 2%, more preferably less than 1.5% by weight based on the totalweight of the modified sulfur cement.

In one preferred embodiment the present invention provides modifiedsulfur concrete which comprises 20-40% by weight of sand, 25-45% byweight of fly ash, 25-45% by weight of elemental sulfur and 0.25-2% byweight of modified sulfur as defined in any one of claims 12 to 15.

Of course, the modified sulfur cement of the present invention and theprocess for its preparation should comply with the internationalstandards ACI 548.2R (Guide for Mixing and Placing Sulfur Concrete inConstruction) and C1159-98R03 (Specification for Sulfur Polymer Cementand Sulfur Modifier for Use in Chemical-Resistant, Rigid SulfurConcrete).

In the process of producing the modified sulfur cement (such as modifiedsulfur concrete) of the present invention, the preferred amounts ofstarting material to use essentially correspond to the amounts that arepreferably present in the modified sulfur cement (such as modifiedsulfur concrete) of the present invention. For example, the process ofproducing modified sulfur concrete typically comprises mixing 20-50% byweight of the elemental sulfur, 50-80% by weight of the aggregate and0.1-0.5% by weight of the modified sulfur, based on the total weight ofthe concrete. In this context, and in other aspects of the presentinvention, the number of significant figures quoted when specifying thepercentage weights of given components within a given composition mustbe borne in mind. Thus, if 80 wt % of aggregate is used, the minimumamount of 20 wt % of elemental sulfur does not preclude the presence ofa small amount (e.g. 0.1 wt %) of modified sulfur. As another example,in another preferred aspect the process of producing the modified sulfurconcrete comprises mixing 20-40% by weight of sand, 25-45% by weight offly ash, 25-45% by weight of elemental sulfur and 0.25-2% by weight ofmodified sulfur.

In the process of producing the modified sulfur cement (such as modifiedsulfur concrete) of the present invention, the mixture of elementalsulfur and modified sulfur (and if necessary the aggregate) can beheated to a temperature of 130-150° C., typically around 140° C., for 30minutes to 2 hours, typically 1 to 1.5 hours.

In another embodiment the process of producing the modified sulfurcement (such as modified sulfur concrete) of the present inventioncomprises mixing together (i) the aggregate which has been pre-heated toa temperature of 170-180° C., typically around 175° C., and (ii) amixture of the elemental sulfur and modified sulfur, which mixture hasbeen pre-heated to a temperature of 130-150° C., typically around 140°C., and then subjecting the mixture of (i) and (ii) to a temperature of130-150° C., typically around 140° C., for 20-40 minutes. The resultingmixture is then typically cast into moulds and allowed to cool.Temperature control is important because modified sulfur cementtypically melts at 119° C. but above 149° C. its viscosity rapidlyincreases to an unworkable consistency.

The process of producing the modified sulfur concrete of the presentinvention can involve mixing the components in different orders.Preferably, the elemental sulfur and modified sulfur are mixed first,and the aggregate is added subsequently. If sand and fly ash are to beused as the aggregate, the fly ash is preferably added before the sand.

Preferably, the modified sulfur cement (such as the modified sulfurconcrete) of the present invention is obtainable by one of theaforementioned processes. In one preferred embodiment the mixture iscast into a particular shape before being cooled, which shape produces ablock of modified sulfur concrete suitable for use a barrier, whichbarrier is suitable for restricting permeation of matter.

When a preparation temperature of 130-140° C. is used to produce themodified sulfur cement of the present invention, this has the advantagethat moisture and other volatile compounds contained in the waste aredriven off, such that small quantities of moisture can be effectivelyvolatilized during the process. The solidification concept of thisprocess is to entrap the aggregates in the sulfur matrix and toimmobilize them physically. Accordingly, in a preferred embodiment themodified sulfur cement of the present invention is modified sulfurconcrete obtainable using a preparation temperature of 130-140° C.

As elemental sulfur for use in the present invention, standard elementalsulfur of any particular form may be used. The elemental sulfur may becommercial grade, crystalline or amorphous. Particle size is generallynot significant and the sulfur may be used as either solid or liquid(molten) form, since the sulfur is melted during the preparation ofsulfur cement.

The use of sulfur is advantageous as it provides a beneficial use forby-products of other industries which are produced at a rate whichexceeds the current market demand. For instance, in the United ArabEmirates (UAE) large quantities of by-product sulfur are currentlygenerated by the cleanup of hydrogen sulfide in the production ofpetroleum and natural gas. This sulfur may be used in accordance withthe present invention.

The sulfur used according to the present invention typically has agranular shape and a purity of 99.9%. It is obtainable from, forinstance, Al Ruwais refinery, UAE.

Containment constructions of the present invention may be produced byhousing one or more barriers of the invention in a containment unit,with appropriately strong support and foundations. Preferably thecontainment construction of the present invention is suitable for use inarid land.

The barriers of the present invention are typically suitable forcontaining matter, such as hazardous waste, over a long time period. Inthis context, “long time period” is intended to reflect the fact thatthe permeation of matter through the barrier is not expected to be alimiting factor on the lifetime of the barrier. It is also intended toreflect the fact that disintegration of the barrier into its surroundingenvironment is not expected to be a limiting factor of the barrier. Inother words, when the barrier is put in place, the functions ofrestricting permeation and minimal disintegration into the surroundingenvironment are expected to continue indefinitely for the lifetime ofthe barrier or for as long as the use is continued.

The long time period may, for instance, be at least 20 years, morepreferably at least 50 years, more preferably still at least 100 years,such as at least 250, 500 or 1000 years. In one preferred embodiment thelong period is essentially indefinite. Thus, typically the structure orconstruction of the present invention is arranged such that it issuitable for restricting permeation indefinitely.

The barrier of the present invention is suitable for containing mattersuch as hazardous waste. The term “suitable for containing matter” isintended to reflect the shape and dimensions of the barrier. Thus, thebarrier of the invention should not have a shape that includes holes orgaps that defeat the object of containing matter. Typically a barrier ofthe present invention will be arranged and shaped so as to surround thematter to be held, with no gaps or holes in the structure in the partsof the barrier that are expected to come into direct contact with thematter to be contained. For instance, a barrier of the present inventionmay be shaped like a cup, flask or bowl, i.e. the sides and base have nogaps or holes and the top has an opening to allow insertion/removal ofthe matter to be contained. Alternatively it could be shaped like a box,cylinder, rod or flat sheet. However, a barrier of the present inventionmight feature a hole or gap in it if, for instance, it is intended torestrict permeation in one or more particular direction(s), in order todirect the fluid matter in another direction.

If the barrier of the present invention is to contain aggregates ashazardous waste (i.e. permeation out of the concrete is beingrestricted) then the shape of the barrier is not important, so long asthe waste aggregates are effectively encompassed within the sulfurconcrete. Of course, if the barrier of the invention is also to restrictpermeation of material which is not part of the barrier itself (i.e.permeation both into and out of the concrete is being restricted) thenthe barrier is preferably arranged and shaped as described above.

Typically the barrier of the present invention is modified sulfurconcrete obtainable or obtained by a controlled process that allows theformation of the modified sulfur concrete mixture into a predeterminedshape. The shape formed in this way must have sufficient structuralintegrity to permit its handling in the subsequent operations withoutcollapse.

Typically a barrier of the present invention is less than 1 m thick, inview of the extremely low hydraulic conductivity of the modified sulfurconcrete of the present invention. Preferably the barrier is 0.3-0.9 m,more preferably 0.5-0.7 m thick.

Typically a barrier of the present invention is a monolith, i.e. asingle solidified block. A containment construction of the presentinvention may comprise one or more barriers of the present invention,though typically just comprises one.

Preferably the barrier of the present invention serves to restrictpermeation across the barrier of matter contained by the barrier. Thus,the barrier protects the surrounding environment from the matter itcontains. However, as well as or instead of this, the barrier may serveto restrict permeation across the barrier of matter from the surroundingenvironment. Thus, the barrier can protect the matter it contains fromthe surrounding environment.

The barriers of the present invention are suitable for restrictingpermeation of matter, such as hazardous waste. By “hazardous waste”, itis meant to refer to matter that could pose a danger due to being e.g.toxic, flammable, and reactive (e.g. oxidising or reducing), anirritant, carcinogenic, corrosive, infectious, teratogenic, mutagenic,explosive or radioactive, or could also refer to matter which has thepotential to easily form hazardous waste. The waste could have a pHranging from e.g. 2-13.

The barriers of the present invention are also suitable for exposure toa marine environment.

It will be clear from the context in which a given barrier orcontainment construction exploits the ability of the modified sulfurconcrete of the present invention to restrict permeation whether or notthat barrier or construction is suitable for containing matter over along time period. For instance, a containment unit intended to househazardous waste for an indefinite period (until or unless some othermeans of using or disposing of it may be found) will be built in such away that reflects its potential permanent existence. For instance, itwould probably be heavy duty and permanently set in position with verysolid foundations. Such a containment unit would be classed as suitablefor use in containing matter over a long time period.

On the other hand, a vat or reaction vessel employed in a factory forproducing chemicals, or a storage tank for temporarily holding achemical, for instance, would not be classed as suitable for containingmatter over a long time period. This would be evident from e.g. the factthat they are not permanently set in position (as they would be expectedto be replaced at some point) and would not have foundations built tolast indefinitely (which would be unnecessary over-engineering given thepurpose). Thus, they would not be built in a manner indicative that theycould potentially be used indefinitely, so would not be suitable forindefinite use.

As already noted, the barriers of the present invention may be used inthe containment of hazardous waste. FIG. 11A shows the design of atypical hazardous waste containment construction. The US EnvironmentalProtection Agency (EPA), for example, requires that the compacted clayliner be at least 0.9 m thick and have a hydraulic conductivity lessthan or equal to 10⁻⁹ m/s. Drainage layers are typically required tohave a hydraulic conductivity greater than or equal to 1 cm/s, and aleak detection system capable of detecting a leak within 24 hours.Flexible membrane liners (FMLs) must be at least 0.76 mm thick.

FIG. 11B shows the design of a typical hazardous waste containmentconstruction that is for use in arid land. The liner consists of a thinlayer of clay sandwiched between two geotextiles or glued to ageomembrane. Various terms have been used to describe this material inthe literature. The general term is double flexible membrane liner(DFML). The design mandates that in arid lands, two layers of DFML mustbe used to protect the ground water beneath the sand substrate. It isworth noting that synthetic materials are expensive, particularly withall the quality control/quality assurance required during construction.Also there is the risk of material puncture leading to the escape ofhazardous leachetes, which could e.g. pollute the ground water bodies.

FIG. 11C shows the design of a new containment construction provided bythe present invention, which is suitable for the containment ofhazardous waste in arid lands. The liner consists of a layer of modifiedsulfur cement/concrete with a minimum thickness of 0.3 m. Such materialshall have a hydraulic conductivity in the order of 10⁻¹³ m/s, which isfar less than the 10⁻⁹ m/s that specified by the US EPA. The liner(modified sulfur cement/concrete) is an inert material with a very lowleaching rate in different environments such as neutral, acidic oralkaline media. It has a good resistance to chemical and physicaldegradation, so retains its strength in different environmentalconditions. The use of this design will incur large savings and protectthe human health and the environment in arid lands. Thus, the presentinvention provides a containment construction comprising one or moreliner layers for restricting the permeation of matter (typicallyhazardous waste), said liner layers being less than 0.9 m thick,typically, less than 0.8 m thick, such as less than 0.7, 0.6 or 0.5 mthick. The minimum thickness is generally 0.3 m.

As has been explained above, the modified sulfur concrete of the presentinvention is particularly advantageous for use in arid land. In thiscontext arid land refers to a land which is temperate, warm or hot, andhas a ratio of annual precipitation to potential evapo-transpiration ofless than 0.65. The modified sulfur concrete of the present invention isalso advantageous for use in lands where the average amount of rainfallrecorded is 10 days or less per year.

The following Examples illustrate the invention.

EXAMPLES

The physical, chemical and mechanical properties of sulfur concretesamples were studied.

Example 1 Preparation of Modified Sulfur

Sulfur modification was achieved by reacting sulfur, a modifying agent(bitumen) and a non-ionic surfactant (Triton X-100) to achieve thedesired linear polysulfide products (which retard sulfurcrystallization). The amount of polysulfide formed based upon the totalamount of polymer present ranged from 1 to 5 wt %, and the reaction timeranged from 45-60 minutes at 140° C. The development of the to reactionwas followed from changes in viscosity and homogeneity of the mixture.

The sulfur used was of a granular shape with purity of 99.9%, obtainedfrom Al Ruwais refinery, UAE.

The modifying agent used was a polymer obtained from Geo-Chem MiddleEast, Dubai, UAE, and physically characterized by a specific gravity of1.0289 g/cm³, a Kinematics viscosity at 135° C. of 431 cSt (431 mm²/s),and a softening point of 48.8° C. It contained 79% carbon, 10% hydrogen,3.3% sulfur and 0.7% nitrogen.

The product, modified sulfur cement, was a mixture of polysulfide andun-reacted elemental sulfur, and possessed glass like properties.Un-reacted free sulfur is generally soluble in CS₂, while polysulfide isinsoluble, although its insolubility depends on the extent ofpolymerisation and also the stirring rate and reaction time.

Thus, the percentage of sulfur present as polysulfide was estimated byexamining the proportion of the reaction products which were extractableusing CS₂. Column chromatography was used to determine the weightaverage molecular weight and number average molecular weight ofpolysulfide. Scanning electron microscopy was used to determine whetheror not the free sulfur crystals were orthorhombic or monoclinic.

The structure of polysulfide, with a % yield of 43%, was confirmed byanalyzing the fraction that was insoluble in CS₂ by columnchromatography (HPLC Agilent 1100; column PLgel Mixed C, 300*7.5 mm*5μm, flow rate of 1 ml/min in chloroform, at room temperature 24° C.).Analysis data indicated the presence of low and high molecular weightfractions of polysulfides with a weight average molecular weight of17417 and an average number molecular weight of 344. Thepoly-dispersability index, which is a reflection of the productmolecular weight distribution, was determined to be 5, confirming thepresence of different polymer fractions.

As a consequence, the rheological properties of the sulfur wereaffected; hence the modified sulfur had a higher viscosity thanunmodified sulfur. This has an important effect on the crystallizationof sulfur. With the more viscous modified sulfur, in which the moleculesare more polymerized, crystal growth is inhibited.

FIGS. 2A and 2B show that on heating of sulfur without polymermodification, alpha (orthorhombic form) sulfur crystals were formed,whereas upon modification of sulfur with a polymer modifying agent, thecrystalline morphology was controlled and the dominant microstructurewas plate like crystals of micron size. Such an interlockedmicrostructure provides ways to relieve stresses that develop duringthermal expansion of sulfur.

Example 2 Preparation of Sulfur Concrete Samples

Sulfur concrete samples were prepared from sulfur, fly ash, sand and themodified sulfur of Example I according to the procedure described in ACI248.2R-93 for mixing and placing sulfur concrete. The freshly preparedconcrete was cast into moulds to form the desired concrete samples.

The sulfur used was of a granular shape with a purity of 99.9%, and wasobtained from Al Ruwais refinery, UAE.

Chemical analysis of the fly ash used (India-97/591) was performed usingInductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES)VISTA-MPX CCD simultaneous. The fly ash mainly consisted of oxides ofsilica (60.9%), aluminum (32.4%) and iron (4.34%) with lesser amounts ofcalcium (0.46%), magnesium (0.66%) and potassium (0.027%). Since thevalue of SAF oxides (i.e., SiO₂+Al₂O₃+Fe₂O₃) was over 50%, it wasclassified as fly ash Type C according to ASTM C 618 (1980).

The sand used was desert sand obtained from a sandy dunes quarry in AlAin area, UAE. Chemical analysis was performed using ICP-AES. The sandmainly consisted of oxides of silica (74.4%), calcium (16.35%) andlesser amounts of magnesium (1.158%), iron (0.676%), aluminum (0.47%),and potassium (0.13%). The sand was screened to obtain grain sizesranging from 0.08 to 0.43 mm.

Samples of sulfur concrete were prepared with dimensions of 50×50×5 mm(cuboid), 50×50×50 mm (cuboid) and 38×77 mm (cylindrical). Setting orhardening of the samples took place on cooling to about 20° C.

Example 3 Characterisation of the Sulfur Concrete Samples

The sulfur concrete samples were subjected to numerous performance teststo determine their physical, chemical, stabilization, solidification,electrical and thermal properties, under anticipated storage anddisposal conditions. Table 1 illustrates the physical and chemicalproperties determined. The samples tested had a high density structurewith a comparable density to hydrated Portland cement. They shouldtherefore provide similar radiation-shielding properties to hydratedPortland cement.

Analysis showed that the best concrete samples were composed of 34.4%sulfur (modified+unmodified), 36.4% fly ash, and 28.95% sand, i.e. theyhad a high aggregate content. However, they also met or exceeded theregulatory and disposal site acceptance criteria.

It was found that the hot mix could be poured at either very high orvery low ambient temperatures without problems (as compared withhydraulic cement concrete, which cannot tolerate such temperaturevariation for the pouring step). Additionally it was found that the hotmix may be maintained in fluid form for many hours withoutdeterioration.

TABLE 1 Physical properties of sulfur concrete, after three days coolingProperty Typical results Density (ASTM C 642) 2210-2370 kg/m³ Settingtime 30-60 minutes Curing Not required Air content (ASTM C 642)  4-8%Max. Moisture absorption (ASTM C 128-97) 0.17% Max. Volumetric shrinkage1.69% Electrical conductivity Nonconductive Max. service temperature85-90° C. Flame spread classification 0 Fuel contributed 0

Example 4 Mechanical Properties of the Sulfur Concrete Samples

It is important for the structures of the present invention to have highimpermeability. Thus, any void spaces (pores) should not be connected,so less water can be absorbed. This phenomenon was prevented thanks tothe sulfur modification which overcame the shrinkage problems associatedwith cooling.

It was found that the samples did not support combustion. Sulfur presentat the surface of the samples burned slowly when exposed to direct flamebut self extinguished when the flame was removed—the low thermalconductivity of sulfur results in slow penetration of heat.

Particular attention should be paid to the high compressive strength ofthe concrete samples (Table 2, for 50×50×50 mm sulfur concrete samples).The samples exhibited mechanical properties greater than Portland cementconcrete. In as little as one hour, 80% of ultimate strength wasachieved, and the samples were usually ready for use in less than oneday. The fast curing property contributes to shortening constructionperiod.

It was found that the concrete prepared according to Example 2 could bepre-cast easily into various shapes.

It was also found that the samples prepared had thermoplasticproperties—they could be crushed, re-melted, re-formed without loss ofstrength or other properties.

TABLE 2 Mechanical properties of sulfur concrete, after three dayscooling Property Typical results Compressive strength (ASTM C 39) 50-54MPa Modulus of Elasticity 1603 MPa Flexural strength Standard EN 196/18.306 N/mm² Maximum load at failure Standard EN 196/1 3.544 KN

Example 5 Hydro-Mechanical Properties of the Sulfur Concrete Samples

The barriers of the present invention may be exposed to aqueousenvironments, and so the hydro-mechanical properties of samples weretested. The exposure of barrier materials to aqueous solutions can causeinternal stresses with resultant cracking and strength losses in thebarrier.

The effect of an aggressive environment on the samples was examined byimmersing concrete samples in 98% sulfuric acid, 50% phosphoric acid,30% boric acid and 10% acetic acid, for 7 days at 24° C. Hydraulicconductivity is the most important property for a containment barrier,as it is a measure of the rate that liquids will penetrate the barrier.Experimentally obtained results of the hydraulic conductivity of thesamples were in the order of 10⁻¹¹-10³ m/s. This indicated that thesamples were impervious to water.

Table 3 summarizes some of the performance data for hydraulicconductivity (for 38×77 mm cylindrical sulfur concrete samples), loss inweights and in mechanical strength (for 50×50×50 mm sulfur concretesamples). Hydraulic conductivity was measured using a flexible membranetest apparatus. The data has revealed that the samples exhibit a highresistance to aggressive environments. It should be noted that under thesame conditions Portland cement concrete, in most of these cases, isdestroyed (McBee et al., Sulfur Construction Materials, Bulletin 678,U.S. Bureau of Mines, Washington D.C., 1985).

TABLE 3 Durability tests; hydraulic conductivity measurements, weightand compressive strength loss of samples after 7 days immersion incorrosive acids Hydraulic Acid type and conductivity Weight LossStrength loss concentration (m/s) (%) (%) water 1.456 × 10⁻¹³ 0.00 0.098% Sulfuric acid 7.660 × 10⁻¹¹ 0.23 13.5 50% Phosphoric acid 3.103 ×10⁻¹² 0.08 7.9 30% Boric Acid 8.176 × 10⁻¹³ 0.07 4.0 10% Acetic acid2.196 × 10⁻¹² 0.14 16.0

Example 6 Durability Performance—28-Day Immersion Test

Concrete samples of Example 1 were immersed in: (a) de-ionized water atdifferent temperatures of 24, 40 and 60° C.; (b) saline solution of 3%NaCl at different temperatures of 40° C. and 60° C., and (c) acidicsolution of 70% sulfuric acid at 40° C. All specimens were immersed fora period of 28 days. At the end of the test period, specimens were thenair-dried in a ventilated hood for 24 hrs. The loss in weight wasdetermined and compared with the values obtained with controlled samples(air), and compressive strength tests were conducted.

Comparison of these samples with companion samples kept in air indicatedthat; there was no observed cracking and dimensional changes werenegligible for all samples. There was no loss in weight and no adverseeffects in compressive strength for samples immersed in water atdifferent temperatures as shown in FIG. 4. Although weight loss insamples immersed in saline solutions was almost insignificant, thesamples showed a small to significant decrease in compressive strengthwith a maximum reduction for the samples that were immersed in 3% salinesolution at 60° C. This could be attributed to the formation of sulfurgas and an increase in the amount of void spaces. Also, it may be theresult of partial detachment between sulfur and the aggregates due tothe presence of sodium chloride.

Example 7 Durability Performance—1 Year Immersion Test

50×50×50 mm modified sulfur concrete samples were tested to determinetheir durability in hydrates and in saline environments after immersionfor one year at room temperature (24±2° C.). Samples were immersedcontinuously for up to 360 days in distilled water, and different salinesolutions of 1% and 3% NaCl concentrations. These samples were comparedwith those kept in air. Periodically, during the course of the test,samples were visually inspected for shrinkage and cracks. In addition,after one year, the sulfur concrete samples were weighed and tested fortheir compressive strength and microstructure.

FIG. 5 shows the compressive strength variations after the samples hadbeen immersed in distilled water and in different saline solutions forone year. The results indicated that sulfur based barriers arecorrosion-resistant and could be used in hydrated and salt environments.No deterioration was observed and only a limited loss in compressivestrength was observed.

A microstructure scan after immersion of specimens for one year indistilled water is shown in FIG. 6. It can be seen that in the presenceof the water there is a tendency for the sulfur which coats theaggregates in the samples to be eluted from the aggregate surfaces.However, the use of modified sulfur cement resulted in an increase inthe resistance of coated sulfur towards this elusion. This could beattributed to the presence of porous bodies impregnated with modifiedsulfur, which may have interactive forces different from those ofordinary sulfur which makes the modified sulfur less susceptible todamage from water (Feldman et al, Cem. Concr. Res., 8, 273-281 (1978);Beaudoin et al, The Int. Journal of Cement Compositions and LightweightConcrete, 6(1), 13-17, (1984)).

FIG. 3 shows how the sulfur concrete samples formed a dense structure,with modified sulfur penetrating deep in between the aggregates byenfolding the aggregates with hydrophobic sulfur.

Example 8 Leaching Tests

The resistance of 50×50×5 mm modified sulfur concrete samples tocorrosive environments was examined by testing samples in variousaqueous environments. The samples were immersed in a transparentcontainer filled with 1000 ml of tested aqueous environment. 1 ml fromeach aqueous solution was used for analysis by ICP for the determinationof the total leaching of sulfur as sulfate, and metals such as calcium,magnesium, aluminium, and iron salts. Each test was run in duplicate toensure reproducibility.

Leaching tests were conducted on concrete samples to evaluate the levelsof environmentally hazardous metal contaminants that could be leachedfrom the structures of the present invention. Chemical leaching ofsulfur (as sulfates) from samples was measured using ICP-AES asdiscussed above. It is worth noting that since elemental sulfur existsin different allotropic forms with different densities, which aresensitive to cooling rates, it may cause micro-cracking and surfaceimperfections that provide excellent spots for oxidation. In thepresence of oxygen and water, sulfur is slowly oxidized to sulfite andthen to sulfate as shown by Eqs. 3 and 4 (Mattus and Mattus, 1994,Evaluation of Sulfur polymer cement as a Waste Form for theImmobilization of Low-Level Radioactive or Mixed Waste, ORNL/TM-12657,Oak Ridge National Laboratory, Oak Ridge, Tenn.).

S⁰+O₂+2H₂O→H₂SO₃  (3)

H₂SO₃+½O₂→H₂SO₄  (4)

The leaching experiments were performed in accordance with theAccelerated Leach Test (ALT) procedures: Monolithic Inorganic Leach TestASTM C 1308. This test method provides a method for accelerating theleaching rate of solidified waste to determine if the release isdiffusion-controlled. This test method is applicable to any materialthat does not degrade, deform, or change leaching mechanism during thetest. If diffusion is the dominant leaching mechanism, then the resultsof this test can be used to model long-term releases from waste forms.The procedures were developed for evaluating the potential leachabilityfrom solidified matrices. The test protocol specifies changes in pHmedium, temperature, surface area to volume ratio, and testing time. Theresults obtained included the incremental and cumulative sulfur fractionleached.

Effect of Time and Solution pH

The leaching tests were run at several constant pH values of 4, 7 and 9to evaluate the influence of pH on the leaching of sulfur and metaloxides from the samples. Universal buffer solutions were used, whichwere prepared by modifying the method reported by Britton, HydrogenIons, 4^(th) Edition Chapman and Hall, 313 (1952), by mixing equalvolumes of acids (acetic acid, phosphoric acid, and boric acid) inbottles. The total molarity of the acid mixture was maintained at 0.4 Mfor the three acids. The desired pHs were reached by mixing the acidmixture with the required amount of 1M sodium hydroxide solution. Aconstant ionic strength of the three buffer solutions—pH 4, 7 and 9 wasmaintained and adjusted using a pH meter. The sulfur concrete sampleswere immersed in a transparent container filled with tested buffersolution. Aliquots were sampled and submitted for ICP analysis todetermine the total amount of sulfur and metals leached.

The incremental leaching data as a function of time are shown in FIGS. 7and 8. The results indicated that:

-   1. The tests carried out have shown that the leaching rates of    sulfur are extremely low irrespective of pH variations in the    aqueous environment. The amount of sulfur leached from the    solidified matrix in acidic (pH 4), neutral (pH 7), and alkaline    (pH 9) mediums is approximately the same, as shown in FIG. 7. This    may suggest that the stability of the solidified matrix in an    aqueous environment is independent of the pH of the solution;    similar results were reported by Sliva et al, “Sulfur Polymer Cement    as a Low Level Waste Glass Matrix Encapsulant”, PNNL-10947, Pacific    Northwest National Laboratory, Richland, Wash., January 1996. A    small increase in the amount of sulfur leached with time was    observed, but the sulfur released could not overcome the buffer    capacity because solution pH was reported to be constant with time.-   2. Since materials such as sands and fly ash contain leachable or    extractable metallic pollutants it is of prime importance to    evaluate the potential leachability of these metal pollutants. The    results shown in FIG. 8 indicated that the main leached metal is Ca,    with lesser amounts of Mg, Al, and Fe also leached. This could be    explained by the electronegativity of these atoms. It is known that    different metals have different tendencies to gain electrons. The    greater the electronegativity of an atom, the greater its affinity    for electrons. The electronegativities of Ca, Mg, Al, Fe, and sulfur    are −1.00, −1.55, −1.61, −1.83, and −2.58, respectively.-   3. It can be seen from FIG. 8 that irrespective of pH values all    curves follow the same trend. Metal ions have lower solubility at    alkaline than acidic pH values. Differences in the basic nature of    the oxides of these metals may explain the different leaching    effects, as discussed below.    -   a. Many metal oxides react with water to form alkaline        hydroxides, e.g., calcium oxide (lime) reacts with water to form        calcium hydroxide.

Metal oxide+Water→Metal Hydroxide  (5)

CaO(s)+H₂O(l)→Ca(OH)₂(aq)  (6)

-   -   b. Some metal oxides do not react with water but are basic when        they react with acid to form salt and water

Metal oxide+Acid→Salt+water  (7)

MgO(s)+2HCl(aq)→MgCl₂+H₂O(l)  (8)

-   -   c. Others exhibit amphoterism, i.e., they react with both acids        and bases, like aluminium oxide which dissolves in a strong acid        and strong base

Al₂O₃+6H⁺→2Al³⁺+3H₂O  (9)

Al₂O₃+6OH⁻+3H₂O→2Al(OH)₆ ³⁻  (10)

-   -   d. Still others are neutral and non-reactive.

-   4. The leaching of the metals increased slightly but linearly with    time throughout the test period, while the solution pH was buffered    at the same pH.

-   5. The leaching of materials is generally very low because of the    low hydraulic conductivity of the solidified matrix. In addition,    because of hardening by solidification, metal oxides found in the    fly ash are chemically bonded within the matrix since they are    converted to less soluble metal sulfides and a small percentage of    sulfates (Darnell et al., Full-scale tests of sulfur polymer cement    and non-radioactive waste in heated and unheated prototypical    containers, EGG-WM-10109, Idaho Natl. Engineering Lab., Idaho Falls,    Id., (1992)). This property of transformation of metal oxides to    less soluble sulfide forms has also been reported by Mayberry et al,    Technical area status report for low-level mixed waste final waste    forms, Vol. 1, DOEMWIP-3, Mixed Waste Integrated Program, Office of    Technology Development, US Dep. Of Energy, Washington D.C. (1993).    These reasons make the sulfur based barriers a good candidate for    utilization as a matrix or binder for the immobilization of wastes.    Leaching studies indicated that the process of modifying the sulfur    minimized or prevented the release of the toxic elements from the    solidified matrix.

Effect of Temperature

Since temperature is an important factor that greatly influences therate of leaching of sulfur and metals from solidified matrix, sampleswere tested in distilled water at temperatures of 24°, 40°, and 60° C.The results shown in FIGS. 9 and 10 highlight the following:

-   1. The leached sulfur for the case of distilled water at room    temperature was of no consequence throughout the test period of 90    days (FIG. 9). This means that the concrete samples were very stable    and insoluble in distilled water at room temperature.-   2. The materials leached from the concrete samples tested in    distilled water were sulfur, Ca, and Mg. Other metals such as Al and    Fe were not detected in the leached products.-   3. The difference in the rate of leaching of materials in distilled    water between 24°, 40° and 60° C., was insignificant during the    early test period up to 19 days, and gradually increased with time;    i.e., the temperature effects were very small and increased slowly    with time. With further increases of immersion time, an expected    increase in sulfur and metal oxides leached into solution was    observed. This was an indication of dependence of the reaction rate    of metals in the solidified matrix on temperature and time when    immersed in distilled water.    The leached rate of metal oxides (Ca and Mg) was insignificant at    room temperature, but slightly enhanced with increased temperature    as shown in FIG. 10. High temperature accelerated the leaching    process because the solubility of metals depends on temperature and    increases consequently as temperature increases (Lageraaen et al,    Use of recycled polymers for encapsulation of radioactive, hazardous    and mixed wastes, BNL-66575, (1997)).    Comparison with Other Cements

Preparation of sulfur cement using elemental sulfur and bitumen, butwithout a non-ionic surfactant, encountered difficulties because theresulting cement was very brittle.

The modified sulfur of the present invention has sulfur present in thebeta (monoclinic) form. Evidence for this is provided in FIG. 12, whichis comparable to FIG. 5 from U.S. Pat. No. 4,391,969, which illustratesthe DSC for modified sulfur prepared using cyclopentadieneoligomr/dicyclopentadiene. However, it should be noted that the modifiedsulfur of the present invention leads to superior stability. Thus,concretes prepared using modified sulfur of the present invention hasbeen found to be stable for a period in excess of two years, whereas themaximum reported storage time for that described in U.S. Pat. No.4,391,969 was six months.

The Examples above demonstrated that the modified sulfur concrete of thepresent invention is from a thermo-mechanical and hydro-chemicalbehaviour point of view, suitable for use as a barrier for restrictingpermeation over a long time period. It could be used for the containmentof hazardous waste in arid lands because of (1) its fast hardening, i.e.less than a day; (2) its high strength, i.e. two to three times that ofPortland cement concrete; (3) its high resistance to acidic, neutral,and alkaline environments; and (4) the very low leachability of metalsfrom the solidified matrix that is observed for it.

What is claimed is:
 1. Modified sulfur which comprises sulfur, a mixtureof oligomeric hydrocarbons and a non-ionic surfactant.
 2. Modifiedsulfur according to claim 1, wherein the degree of polymerisation in themodified sulfur is at least 10%.
 3. Modified sulfur according to claim1, wherein the surfactant is an alkylphenoxy polyethoxy ethanol. 4.Modified sulfur according to claim 3, wherein the alkylphenoxypolyethoxy ethanol has the average formulaC_(r)H_(2r+1)(C₆H₄)O(CH₂CH₂O)_(s)CH₂CH₂OH, wherein r is from 4 to 8 ands is from 7 to
 40. 5. Modified sulfur according to claim 4, wherein, inthe average formula, r is 8 and s is
 9. 6. Modified sulfur according toclaim 1, wherein the surfactant is iso-octylphenoxy polyethoxy ethanol.7. Modified sulfur according to claim 1, wherein the chemical structureof the surfactant is

wherein x is 9 to
 10. 8. Modified sulfur according to claim 1, whereinthe mixture of oligomeric hydrocarbons is bitumen.
 9. Modified sulfuraccording to claim 1, wherein the sulfur component comprises 45-65% byweight of monoclinic sulfur and 35-55% by weight of polysulfide, basedon the total weight of the sulfur component.
 10. Modified sulfuraccording to claim 1, wherein the mixture of oligomeric hydrocarbons isbitumen, and the modified sulfur comprises 95-97.5% by weight of sulfur,and 2.5-5% by weight of the total of bitumen and surfactant components,based on the total weight of the modified sulfur.
 11. Modified sulfuraccording to claim 1, which is obtainable by a process that comprisesmixing elemental sulfur, a mixture of oligomeric hydrocarbons and anon-ionic surfactant to produce a mix, and wherein the process comprisessubjecting the mix to a temperature of 120-150° C. for 30 minutes to 3hours.
 12. Modified sulfur according to claim 11, wherein the mix coolsat a rate of about 1° C. per minute after being subjected to thetemperature of 120-150° C. for 30 minutes to 3 hours.
 13. Modifiedsulfur cement, which comprises modified sulfur as defined in claim 1 andelemental sulfur.
 14. Modified sulfur concrete, which comprises modifiedsulfur as defined in claim 1, elemental sulfur and an aggregate. 15.Modified sulfur concrete according to claim 14, wherein the aggregatecomprises fly ash and/or sand.
 16. Modified sulfur concrete according toclaim 15, which comprises 20-40% by weight of sand, 25-45% by weight offly ash, 25-45% by weight of elemental sulfur and 0.25-2% by weight ofmodified sulfur as defined in claim
 1. 17. Modified sulfur concreteaccording to claim 14, which is obtainable by a process of mixing anaggregate with the elemental sulfur and modified sulfur to produce amixture.
 18. Modified sulfur concrete according to claim 17, wherein theprocess comprises mixing 20-50% by weight of the elemental sulfur,50-80% by weight of the aggregate and 0.1-0.5% by weight of the modifiedsulfur, based on the total weight of the concrete.
 19. Modified sulfurconcrete according to claim 17, wherein the process comprises subjectingthe mixture to a temperature of 130-150° C. for 30 minutes to 2 hours.20. Modified sulfur concrete according to claim 17, wherein the processcomprises mixing together (i) the aggregate which has been pre-heated toa temperature of 170-180° C., and (ii) a mixture of the elemental sulfurand modified sulfur, which mixture has been pre-heated to a temperatureof 130-150° C., and then subjecting the mixture of (i) and (ii) to atemperature of 130-150° C. for 20-40 minutes, casting the resultingmixture into molds and allowing it to cool.